The present invention relates to a method of casting with pouring from the bottom (ascending casting) with post-feeding and to a casting mould (or mould part), said method and casting mould being of the kind in which regions of reduced pressure formed during the cooling are fed with melt.
It is commonly known that metals, when cooled from the liquid to the solid state, undergo a reduction in volume, a so-called thermal contraction. In casting moulds, in which a non-uniform heat distribution reigns in the mould cavity after the pouring, and in which for this reason all parts of the casting do not solidify at the same time, this causes the parts of the casting solidifying last to give off liquid metal to compensate for the contraction of the parts of the casting having solidified earlier, possibly leading to faults in the casting, commonly known as xe2x80x9cshrinkage holesxe2x80x9d appearing in the form of the presence in the surface of the casting, or cavities (macroscopic or microscopic holes) within the casting. In order to avoid these casting faults, the skilled person can take recourse to a series of expedients, of which the most common is the use of feeding reservoirs, i.e. cavities in the mould being filled with metal during the pouring and having such dimensions that the metal in them solidifies later than the parts of the casting solidifying last, being connected to the latter through ducts having a relatively large cross-sectional area, thus being able to post-feed these parts with liquid metal to compensate for the contraction.
Such post-feeding reservoirs are mainly known in two forms, viz. as feeders or risers, i.e. substantially cylindrical cavities leading from the duct connecting them to the casting to the upper surface of the mould, or in the form of internal or closed cavities in the mould, so-called xe2x80x9cblind feedersxe2x80x9d or xe2x80x9cshrinkage knobsxe2x80x9d placed in the immediate vicinity of the part of the casting to be post-fed. Compared to the latter type, the former presents the advantage that the highest metallostatic pressure at the feeding location, i.e. the pressure from the superjacent metal column, to a high degree assists the feeding by pressing the feeding metal through the connecting duct into the casting, in contrast to which the pressure in the latter type diminishes during the feeding process. On the other hand, the latter type presents the advantage of normally producing a higher yield of metal in the casting process, i.e. a lesser quantity of metal to be separated from the casting after the casting process for subsequent re-melting (re-circulation), which also reduces the energy used for melting.
When risers or xe2x80x9cshrinkage knobsxe2x80x9d connected to the mould cavity proper are used, they are conventionally filled with melt having been cooled during the pouring process, which is especially the case with bottom pouring. For this reason, these cavities constituting the post-feeding reservoirs must be made sufficiently large to ensure thatxe2x80x94in spite of the coolingxe2x80x94liquid melt is still present in the reservoir for post-feeding when the casting solidifies. The result of this can be that, when using certain alloys or when producing critical castings, a yield of only approximately 50% can be achieved, i.e. that after the casting, the post-feeders and the ingate system weigh the same as the casting to be produced. The amount of material thus being necessary to melt in addition to what is used for the desired casting itself constitutes an energy loss, increasing the cost of the casting process and at the same time necessitating a higher melting capacity for the foundry equipment.
Some of these disadvantages can be avoided by constructing and using the ingate system as a post-feeding means, to this end comprising post-feeding cavities. In this manner, a post-feeding reservoir is obtained that is heated by the melt on the latter""s passage to the mould cavity. Optimally, this post-feeding reservoir must be constructed to have the least possible heat loss, so that the least possible quantity of melt is used for heating the reservoir and maintaining it hot so as to maintain the melt in it in the liquid state. The least possible heat loss is i.a. achieved by constructing the reservoir with the least possible surface area per unit of heat. Further, the heat loss is minimized during the post-feeding process by placing the reservoir close to the mould cavity. The total result of this is that such post-feeding reservoirs, in consideration of the heat loss, are optimally constructed as cavities constituting a large widened part of the ingate system immediately upstream of the inlet to the mould cavity. When using bottom ingates, this does, however, give rise to the disadvantage that the bottom ingate must be shaped and arranged in such a manner that it will not be blocked by solidified melt until the casting itself is solidified to such a degree that post-feeding is no longer necessary.
From DE-36 21 334 it is known to use a movable tube for pouring. The post-feeding is provided by a riser positioned at the top of the casting cavity.
DE-34 44 941 shows another possibility using a movable tube for pouring. The post-feeding is provided from a post-feeding reservoir at the side of the casting cavity, said reservoir being connected to the casting cavity at the bottom via the ingate, which means that the post-feeding is performed through the bottom-filling inlet to the casting cavity, whereby said bottom-filling inlet will have to be constructed in such a manner that it will not be blocked by solidified melt until the casting itself is solidified to such a degree that post-feeding is no longer necessary.
It is, on this background, the object of the present invention to provide a method and casting mould of the kind referred to initially, with which it can be achieved
that the inlet from the post-feeding reservoir is not blocked by solidified material before the casting itself has solidified to a degree not necessitating additional post-feeding,
that the surplus material to be removed after the casting process is kept at the lowest level possible,
that the ingate system, including the post-feeding reservoir, occupies the least possible space in the mould, and
that the position of the post-feeding can be selected more freely.
This object is achieved with a method of the kind referred to initially, being characterized by the use of an ingate system comprising a downsprue extending downward from an inlet to an outlet communicating with the bottom ingate of the casting cavity and connecting to a feeding reservoir through flow-restricting means, from which feeding reservoir a feeding duct extends to a location in the side of the casting cavity at least approximately at the same level as the level of the region of reduced pressure in the casting cavity in which the solidification can be expected to take place last.
The invention is based upon the fact that, when the post-feeding takes place through a duct debouching close to the place, where the solidificationxe2x80x94on the basis of experience and/or calculationsxe2x80x94can be expected to take place last, also called the thermal centre of gravity for the casting, the un-solidified melt in the casting and the post-feeding reservoir co-operate to keep the post-feeding inlet duct open, providing the advantage that it is not necessary to use melt for filling a bottom ingate system and keep the latter heated, making it possible to arrange and construct the bottom ingate system primarily with regard to the flow conditions and minimized material consumption and to a lesser degree in consideration of late solidification. Further, the column of melt and the pressure connected to and possibly applied to same may post-feed melt to the mould cavity during the post-feeding process with a minimum of friction.
The present invention also relates to a casting mould or mould part for use when carrying out the method according to the invention. This casting mould or mould part is of the kind set forth above and in detail hereinbelow.
Additional advantageous embodiments of the method and the casting mould or mould part, the effects of whichxe2x80x94beyond what is obviousxe2x80x94are explained in the following detailed part of the present description.